Mix the sauce ingredients in a medium saucepan over low heat. Stir periodically.

In a separate pan, boil enough water to cover the fish. While it is boiling, add 2 slices of lemon and the salt. Bring back up to a boil, then remove from heat and add fish. Cover and cook on lowered heat until the fish is flaky, anywhere from 8-15 minutes.

While the fish is cooking, cook down the butter and oil in a separate saucepan. Add the onions and sugar. Cook covered approximately ½ hour, or until the onions are limp and clear. Add the spinach just before serving, stirring them into the onions until they are wilted.

Serve with the salmon resting on a bed of onions and spinach, topped with a line of sauce.